Activated carbon, method for production thereof, polarizing electrode and electrical double layer capacitor

ABSTRACT

In activated carbon obtained by subjecting a carbonaceous material to an activation treatment, the overall content of alkali metals is set at 100 ppm or less, or the overall content of heavy metals is set at 20 ppm or less and the overall content of alkali metals is set at 200 ppm or less. In cases where such activated carbon is used as a raw material in electronic devices, the formation of dendrites by the reductive deposition of alkali metals or heavy metals tends not to occur, so that problems such as short-circuiting or the like tend not to arise, and a good rate of self-discharge retention is shown.

TECHNICAL FIELD

The present invention relates to activated carbon, a method formanufacturing the same, a polarizing electrode and an electrical doublelayer capacitor. In addition to applications such as washing, cleaning,recovery, gas storage, decoloring and the like that utilize theadsorption performance of the activated carbon of the present invention,the activated carbon of the present invention is suitable for use inelectrical devices that utilize the characteristic low content of alkalimetals and/or heavy metals in the activated carbon, especially inelectrodes for secondary batteries and electrical double layercapacitors.

BACKGROUND ART

Activated carbon is widely used in the food industry, chemical industry,pharmaceutical industry and various other industries; in concrete terms,examples of use include applications such as clean water manufacturingapplications, air cleaning applications, solvent recovery applications,stack effluent desulfurization and denitrogenization applications,decoloring applications, tap water treatment, sewage treatment,treatment of feces and urine, industrial waste water treatment, sugarrefining applications, nuclear power applications (adsorption ofradioactive substances), methane occlusion, hydrogen occlusion and thelike. These applications mainly utilize the adsorption performance ofactivated carbon. But, the activated carbon as an adsorbing agent thatis more superior in terms of adsorption performance is desired.

In recent years, meanwhile, electrical double layer capacitors haveattracted attention as back-up power supplies, auxiliary power suppliesand the like, and development focusing on the performance of activatedcarbon as electrodes used in electrical double layer capacitors has beenwidely undertaken. Electrical double layer capacitors which useactivated carbon in polarizing electrodes are superior in terms ofelectrostatic capacitance; accordingly, along with development in thefield of electronics, there has been a rapidly growing demand inelectronic device electrode applications and the like. Recently,furthermore, in addition to miniaturization in conventional memoryback-up power supplies and the like, there has also been development ofhigh-capacitance products used in the auxiliary power supplies of motorsand the like.

Among these fields of utilization of activated carbon, activated carbonthat contains no alkali metals or heavy metals is preferable in thefields of food products, drug manufacture, clean water and electronicdevices. Conventionally, therefore, methods for manufacturing activatedcarbon have generally been methods in which a carbonaceous material issubjected to a gas activation treatment or a chemical activationtreatment, e.g., an alkali activation treatment using an alkali metalhydroxide as an activation assistant, and the system is then washed witha strong acid such as hydrochloric acid, nitric acid, sulfuric acid orthe like in order to remove alkali metals and heavy metals from theproduct of the activation treatment.

However, in cases where activated carbon washed with a strong acid isused as a raw material in electronic devices, e.g., an electrodematerial in non-aqueous electrolyte secondary batteries or electricaldouble layer capacitors, problems such as shorting and the like occur asa result of dendrite formation caused by the reductive deposition ofalkali metals and heavy metals; furthermore, self-discharging tends tooccur as a result of alkali metal ions and heavy metal ions, so that theproblem of a low rate of electrostatic capacitance retention due toself-discharging is encountered (below, the rate of electrostaticcapacitance retention due to self-discharging will be abbreviated to“the rate of self-discharge retention”).

Especially in the case of an alkali activation treatment using an alkalimetal hydroxide as an activation assistant, since the alkali metalhydroxide is strongly oxidizing compound, corrosion of the heatingfurnace used for activation occurs during the activation treatment, andheavy metals are admixed with the product of the activation treatment,so that even if this product of the activation treatment is washed withhydrochloric acid or nitric acid, it is extremely difficult tomanufacture activated carbon from which heavy metals have beencompletely eliminated. If activated carbon containing admixed heavymetals is used as a raw material in electronic devices, e.g., as a rawmaterial for polarizing electrodes in an electrical double layercapacitor, heavy metal particles formed into dendrites will be formed onthe separators of such electrical double layer capacitors by thereductive deposition of heavy metals as described above, so thatproblems such as the opening of holes in the separators or the likeoccur, thus leading to trouble such as short-circuiting or the like.Furthermore, there may be cases in which alkali metals originating inthe alkali metal hydroxides used as activation reagents remain in theactivated carbon. If such activated carbon is used as a raw material forpolarizing electrodes in electrical double layer capacitors, the leakagecurrent is increased, so that the charging efficiency drops, thusresulting in a poor energy efficiency (in other words, the rate ofself-discharge retention drops).

For example, an electrical double layer capacitor using activated carbonwith an Fe content of 200 ppm or less, a Cr content of 10 ppm or less,an Ni content of 10 ppm or less, an Na content of 200 ppm or less, a Clcontent of 300 ppm or less and an ash content of 0.5% or less as apolarizing electrode material has been proposed in Japanese PatentApplication Laid-Open No. 1-241811. In this publication, it is indicatedthat metal components are admixed in the manufacturing process of theactivated carbon, and that the elution of these metal components causesa drop in the long-term reliability of the electrical double layercapacitor. However, there is no description of how to suppress suchmetal contents in order to realize a reliable electrical double layercapacitor, although it is described that the elution of these metalcomponents causes a drop in the long-term reliability of the electricaldouble layer capacitor.

Furthermore, activated carbon for use in the polarizing electrodes ofelectrical double layer capacitors which allows the use of an alkaliactivation process, and which is manufactured by washing with water, anacid solution and then an alkali solution following activation, isdisclosed in Japanese Patent Application Laid-Open No. 2002-43190, andit is indicated that self-discharge can be reduced by reducing the Nicontent. However, an electrical double layer capacitor with desiredperformance cannot be constructed merely by reducing the Ni content.

Accordingly, it is an object of the present invention to provideactivated carbon which does not lead to dendrite formation by thereductive deposition of alkali metals or heavy metals when used as a rawmaterial in electronic devices, so that problems such asshort-circuiting tend not to occur, and which shows a high rate ofself-discharge retention, so that this activated carbon is suitable foruse in applications such as electronic devices and the like.

DISCLOSURE OF THE INVENTION

The present inventors discovered that the abovementioned problemsencountered in the prior art are caused by excessive overall contents ofalkali metals and/or heavy metals in activated carbon, and adjusting theoverall contents of alkali metals and/or heavy metals in activatedcarbon to specified numerical values or less is therefore an effectivemeans of preventing the occurrence of the abovementioned problems.Furthermore, the present inventors also discovered that the reason forsuch excessive overall contents of alkali metals and/or heavy metals inactivated carbon is as follows: specifically, in cases where activatedcarbon (or the product of an activation treatment) is washed with astrong acid, alkali metal salts and/or heavy metals remaining in theform of hydroxides or the like (e.g., nickel hydroxide, copperhydroxide, zinc hydroxide and the like) which have a high affinity foractivated carbon and a relatively low solubility in water are difficultto be removed from the activated carbon, so that considerable amounts ofalkali metals and/or such heavy metals remain in the activated carbon.Furthermore, the present inventors also discovered that (i) alkalimetals can be removed from activated carbon by using carbonic acid toconvert these alkali metals into carbonates, thus lowering the affinitybetween the alkali metals and activated carbon and increasing the watersolubility of the alkali metals, (ii) heavy metals can be removed fromactivated carbon by using basic substances to convert the heavy metalsinto complexes, thus lowering the affinity between the heavy metals andactivated carbon and increasing the water solubility of the heavymetals, (iii) heavy metals can be removed by washing the product of anactivation treatment with an acidic aqueous solution containing anoxidizing agent, and (iv) alkali metals (hydroxides, carbonates and thelike) can be removed more efficiently in cases where such an activationtreatment product is washed with hot water than in cases where thisproduct is washed with water at an ordinary temperature; moreover, heavymetals (hydroxides and the like) can be efficiently removed by washingsuch a product with hot hydrochloric acid. The present inventorsperfected the present invention on the basis of these findings.

Specifically, the first invention of the present application providesactivated carbon which is obtained by subjecting a carbonaceous materialto an activation treatment, wherein the overall content of alkali metalsin the activated carbon is 100 ppm or less. Furthermore, the firstinvention of the present application provides a method for manufacturingactivated carbon, comprising by subjecting a carbonaceous material to anactivation treatment, and then washing the activation treatment productthus obtained with a liquid containing a carbonic acid to give theactivated carbon.

The second invention of the present application provides activatedcarbon which is obtained by subjecting a carbonaceous material to anactivation treatment, wherein the overall content of heavy metal in theactivated carbon is 20 ppm or less. Furthermore, the second invention ofthe present application provides a method for manufacturing activatedcarbon, comprising subjecting a carbonaceous material to an activationtreatment, and then washing the activation treatment product thusobtained with a liquid containing a basic substance to give theactivated carbon.

The third invention of the present application provides activated carbonin which the effect of the invention is heightened by specifying thecarbonaceous material in the second invention as an easily graphitizablecarbonaceous material and specifying the activation treatment as analkali activation treatment, that is, provides an activated carbonobtained by subjecting an easily graphitizable carbonaceous material toan alkali activation treatment, wherein in the activated carbon, theoverall content of heavy metals is 20 ppm or less, and the overallcontent of alkali metals is 200 ppm or less. Furthermore, the thirdinvention of the present application provides a method for manufacturingactivated carbon, comprising subjecting an easily graphitizablecarbonaceous material to an alkali activation treatment, and washing theactivation treatment product thus obtained with an acidic aqueoussolution that contains an oxidizing agent, to give the activated carbon.Furthermore, the third invention of the present application provides amethod for manufacturing activated carbon, comprising subjecting aneasily graphitizable carbonaceous material to an alkali activationtreatment, and then washing the activation treatment produced thusobtained with hot water, hot hydrochloric acid and water in that order,or washing this product with hot water, carbonated water, hothydrochloric acid, aqueous ammonia and hot water in that order, orwashing this product with hot water, carbonated water, hot hydrochloricacid, aqueous ammonia, hot hydrochloric acid and hot water in thatorder, to give the activated carbon.

Furthermore, the fourth invention of the present application provides apolarizing electrode which is formed by mixing the activated carbon ofthe first, second or third invention of the present application with atleast a binder and a conductive material, and also provides anelectrical double layer capacitor using this polarizing electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional schematic diagram which shows one example of theelectrical double layer capacitor of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

First, the first invention of the present application will be described.

The activated carbon of the first invention of the present applicationis obtained by subjecting a carbonaceous material to an activationtreatment. Here, the overall content of alkali metals in the activatedcarbon of the present invention is 100 ppm or less. The reason for thisis as follows: specifically, if the overall content of alkali metalsexceeds 100 ppm, the formation of metal dendrites by reductivedeposition will occur in cases where the activated carbon is used as araw material in electronic devices, thus leading to problems such asshort-circuiting and the like, and causing a drop in the rate ofelectrostatic capacitance retention due to metal ions.

In the first invention of the present application, examples of alkalimetals that may be contained in the activated carbon include lithium,sodium, potassium and cesium. Ordinarily, sodium and/or potassium showlarge contents, and it is important to control these contents. In somecases, furthermore, alkali metals may be present as metallic alkalies inthe activated carbon; ordinarily, however, these alkali metals arepresent in the form of oxides, hydroxides, chlorides or the like.

The overall content of alkali metals in the activated carbon can bemeasured by atomic absorption analysis.

Means of reducing the overall content of alkali metals in the activatedcarbon of the first invention of the present application to 100 ppm orless include repeated washing with ion exchange water, strongly acidicaqueous solutions or the like. To the means, the activated carbonmanufacturing method of the present invention, which is characterized inthat washing is performed using a liquid containing carbonic acid(described later), is preferably applied.

There are no particular restrictions on the carbonaceous raw materialused in the first invention of the present application, as long as thismaterial can be converted into activated carbon by carbonization and/oractivation. Examples of materials that can be used include plant-derivedmaterials, e.g., wood, sawdust, coconut shells, nut shells (charcoal)such as walnut shells or the like,-fruit pits (charcoal), lignin and thelike, mineral materials such as peat, peat moss, lignite, brown coal,bituminous coal, anthracite, coke, coal tar, coal pitch, petroleumdistillation residue, petroleum pitch and the like, natural materialssuch as cotton, rayon and the like, and synthetic materials such asphenol resins, acrylics, vinylon and the like.

Furthermore, there are no particular restrictions on the activationtreatment; conventional universally known gas activation treatments andchemical activation treatments may be used.

The activated carbon of the first invention of the present applicationdescribed above can be manufactured in a preferable manner by theactivated carbon manufacturing method of the first invention of thepresent application, which will be described below.

Specifically, the activated carbon manufacturing method of the firstinvention of the present application is a method in which activatedcarbon is obtained by subjecting a carbonaceous material to anactivation treatment, and then washing the activation treatment productthus obtained with a liquid containing carbonic acid (carbonic acidwashing treatment). As a result of the activation treatment productbeing subjected to a carbonic acid washing treatment, the alkali metalscontained in the activated carbon are converted into alkali carbonatesthat show a high water solubility; accordingly, the alkali metal contentremaining in the activated carbon can be reduced to 100 ppm or less.

The “liquid containing carbonic acid” used in the carbonic acid washingtreatment is preferably carbonated water. There are no particularrestrictions on the concentration of carbonic acid in this carbonatedwater; ordinarily, however, this concentration is 0.1 wt % to 10 wt %.

Furthermore, there are no particular restrictions on the amount of theabovementioned “liquid containing carbonic acid” that is used in thecarbonic acid washing treatment. In practical terms, an amount rangingfrom 1 to 100 parts by weight per part by weight of the activationtreatment product that is to be washed is preferable; in particular, ifthe operating characteristics and alkali metal removal efficiency aretaken into consideration, an amount ranging from 2 to 50 parts by weightis more preferable, and an amount ranging from 5 to 20 parts by weightis especially preferable.

There are no particular restrictions on the temperature during thecarbonic acid washing treatment. However, if the temperature is toohigh, the carbonic acid concentration in the “liquid containing carbonicacid” will drop, so that the alkali metal removal efficiency shows anexcessive drop. On the other hand, if the temperature is too low, thealkali metal removal efficiency will drop in terms of the reaction rate.Accordingly, the temperature is preferably 0 to 50° C., and is even morepreferably 0 to 45° C.

Furthermore, in the carbonic acid washing treatment, there are noparticular restrictions on the number of times that the activationtreatment product is washed with the “liquid containing carbonic acid”.However, the number of times that this washing is performed isordinarily 1 to 3 times in accordance with the quantities of alkalimetals remaining in the activated carbon and the desired alkali metalconcentration levels.

There are no particular restrictions on the pressure that is appliedwhen the activation treatment product is washed with the “liquidcontaining carbonic acid” in the carbonic acid washing treatment.However, if this pressure is too high, special pressure-applying meansmust be installed in the washing apparatus; accordingly, the pressure ispreferably atmospheric pressure to 1 MPa, and is even more preferablyatmospheric pressure to 0.5 MPa.

After activated carbon is obtained by washing the activation treatmentproduct with the “liquid containing carbonic acid” as described above(carbon acid washing treatment), if necessary, it is preferable that ahydrochloric acid washing treatment be performed to remove heavy metals(e. g., nickel, copper, zinc or the like) contained in very smallamounts in the activated carbon.

There are no particular restrictions on the concentration of thehydrochloric acid used in the hydrochloric acid washing treatment. Theconcentration of hydrochloric acid that is generally commerciallymarketed is sufficient. Accordingly, it is preferable that theconcentration of the hydrochloric acid used be 0.01 to 37 wt %; inparticular, if the operating characteristics and safety are taken intoaccount, a concentration of 0.1 to 30 wt % is even more preferable, anda concentration of 0.5 to 25 wt % is especially preferable.

There are no particular restrictions on the temperature at which thehydrochloric acid washing treatment is performed. However, if thistemperature is too high, the hydrochloric acid tends to volatilize, andif this temperature is too low, the heavy metal removal efficiency maydrop. Accordingly, this temperature is preferably in the range of 10 to90° C., and is even more preferably in the range of 20 to 90° C.

Furthermore, there are no particular restrictions on the number of timesthat the hydrochloric acid washing treatment is performed. However,although this number of times also depends on the amounts of alkalimetals that remain in the activated carbon, and the desired alkali metalconcentration levels, the number of times that the treatment isperformed is generally 1 to 3 times.

It is preferable that the activated carbon that has been subjected tothe above-mentioned hydrochloric acid washing treatment then besubjected to a clean water washing treatment using distilled water orion exchange water. Activated carbon obtained by performing a carbonicacid washing treatment may also be immediately subjected to a cleanwater washing treatment without performing a hydrochloric acid washingtreatment.

There are no particular restrictions on the amount of distilled water orion exchange water that is used in the clean water washing treatment.However, it is preferable that washing be performed until chlorine ionscan no longer be detected. In concrete terms, it is preferable that theamount of water used be in common 1 to 10,000 parts by weight per partby weight of activated carbon, although this also depends on the type ofclean water washing system used as described later), and if economy andoperating characteristics are taken into account, an amount ranging from1 to 1000 parts by weight is even more preferable.

Examples of clean water washing systems that can be used include systemsin which washing while agitating is performed using a tank equipped withan agitator, systems in which washing is performed by using a filter andpassing the liquid through under reduced pressure or pressurization, orthe like.

The activated carbon that is obtained by washing with a liquidcontaining carbonic acid, or the activated carbon obtained by furtherperforming a hydrochloric acid washing treatment and/or a clean waterwashing treatment if necessary, is dried under heating and/or reducedpressure, thus producing activated carbon in a dry state.

Furthermore, in the activated carbon manufacturing method of the firstinvention of the present application, a carbonization treatment using anordinary method in accordance with the type of carbonaceous materialinvolved (e.g., a dry distillation treatment at 400 to 800° C.) may beperformed prior to the activation treatment if necessary.

In the manufacturing method of the first invention of the presentapplication, a carbonaceous material is subjected to an activationtreatment. There are no particular restrictions on the treatment methodused; a conventional universally known gas activation treatment methodor chemical activation treatment method may be used.

For instance, activation treatments using steam, carbon dioxide, oxygen,propane combustion waste gas, mixed gases consisting of these gases orthe like may be cited as examples of gas activation treatments, andactivation treatments using chemicals such as zinc chloride, calciumchloride, phosphoric acid, sulfuric acid, sodium hydroxide, potassiumhydroxide, magnesium hydroxide, calcium hydroxide or the like may becited as examples of chemical activation treatments. In particular, incases where a general alkali activation treatment using potassiumhydroxide or sodium hydroxide as a chemical agent is performed, alkalimetals tend to remain in the activated carbon, since an alkali metalhydroxide is used as the activation assistant. Accordingly, in caseswhere a common alkali activation treatment is performed, themanufacturing method of the first invention of the present applicationcan be applied in a preferable manner.

Next, the second invention of the present application will be described.

The activated carbon of the second invention of the present applicationis activated carbon which is obtained by subjecting a carbonaceousmaterial to an activation treatment, wherein the overall content ofheavy metals in activated carbon is 20 ppm or less. The reason for thisis as follows: specifically, if the overall content of heavy metalsexceeds 20 ppm, then dendrite formation caused by the reductivedeposition of heavy metals will occur in cases where the activatedcarbon is used as a raw material in electronic devices, thus leading toproblems such as short-circuiting and the like, and resulting in a dropin the rate of self-discharge retention. Furthermore, in cases where theactivated carbon is used (for example) as an adsorbent material in themanufacture of clean water, heavy metals will continue to be eluted(though in small amounts) in the clean water that is obtained.

In the second invention of the present application, nickel, copper,zinc, iron, silver and the like may be cited as examples of heavy metalscontained in the activated carbon. Among these, the metals thatordinarily show large contents are nickel, copper, zinc and iron, and itis important to control the contents of these metals. In particular, itis preferable that the nickel content be controlled to 8 ppm or less,that the copper content be controlled to 1 ppm or less, that the zinccontent be controlled to 1 ppm or less, that the iron content becontrolled to 0.3 ppm or less, and that the silver content be controlledto 0.1 ppm or less.

The contents of heavy metals in the activated carbon can be measured byinductively coupled plasma emission spectrometry analysis (ICP).

Repeated washing with, e.,g., ion exchange water or a strongly acidicaqueous solution may be cited as means of reducing the overall contentof heavy metals in the activated carbon of the second invention of thepresent application to 20 ppm or less; preferably, however, to themeans, the activated carbon manufacturing method of the presentinvention, which is characterized in that washing is performed with aliquid containing a basic substance (described later), is applied.

The carbonaceous raw materials used in the second invention of thepresent application are as described in the first invention of thepresent application.

Furthermore, there are no particular restrictions on the activationtreatment used; conventional universally known gas activation treatmentmethods and chemical activation treatment methods may be used.

The above-mentioned activated carbon of the second invention of thepresent application can be preferably manufactured by the activatedcarbon manufacturing method of the second invention of the presentapplication, which will be described below.

Specifically, the activated carbon manufacturing method of the secondinvention of the present application is a method in which activatedcarbon is obtained by subjecting a carbonaceous material to a gasactivation treatment or chemical activation treatment, and then washingthe activation treatment product thus obtained with a liquid containinga basic substance (basic washing treatment). As a result of theactivation treatment product thus being subjected to a basic washingtreatment, the heavy metals contained in this product form complexeswhich show a high water solubility; accordingly, the content of heavymetals remaining in the activated carbon can be reduced to 20 ppm orless.

The “liquid containing a basic substance” that is used in the basicwashing treatment is preferably an aqueous solution in which a basicsubstance such as ammonia, organic amine, ammonium carbonate, a mixtureof these substance or the like is dissolved in water. Here, from thestandpoint of ease of removal of the basic substance from the activatedcarbon, ammonia, an organic amine or a mixture of these substances ispreferable as the above-mentioned basic substance. Examples of organicamines include methylamine, ethylamine, propylamine, dimethylamine,diethylamine, dipropylamine, trimethylamine, triethylamine and the like.

Furthermore, there are no particular restrictions on the concentrationof the basic substance contained in the “liquid containing a basicsubstance”; ordinarily, however, this concentration is 0.1 to 10 wt %.

Furthermore, there are likewise no particular restrictions on the amountof the “liquid containing a basic substance” that is used in the basicwashing treatment. In practical terms, an amount ranging from 1 to 100parts by weight per part by weight of the activation treatment productthat is to be washed is preferable; in particular, if the alkali metalremoval efficiency is taken into account, an amount ranging from 2 to 50parts by weight is more preferable, and an amount ranging from 5 to 20parts by weight is especially preferable.

There are no particular restrictions on the temperature during the basicwashing treatment. However, if this temperature is too high, the basicsubstance tends to volatilize from the “liquid containing a basicsubstance”, and if this temperature is too low, the heavy metal removalefficiency drops in terms of the reaction rate. Accordingly, thistemperature is preferably 10 to 60° C., and is even more preferably 20to 50° C.

Furthermore, there are no particular restrictions on the number of timesthat the activation treatment product is washed with the “liquidcontaining a basic substance” in the basic washing treatment; however,this number of times is ordinarily 1 to 3 times in accordance with theamounts of heavy metals remaining in the activated carbon and thedesired heavy metal concentration levels.

There are no particular restrictions on the pressure that is appliedwhen the activation treatment product is washed with the “liquidcontaining a basic substance” in the basic washing treatment. However,if this pressure is too high, special pressure-applying means must beinstalled in the washing apparatus. Accordingly, this pressure ispreferably a pressure ranging from atmospheric pressure to 1 MPa, and iseven more preferably a pressure ranging from atmospheric pressure to 0.5MPa.

After activated carbon is obtained by washing the activation treatmentproduct with the “liquid containing a basic substance” (basic washingtreatment) as described above, it is preferable that this product befurther washed with hydrochloric acid in order to remove the basicsubstance if necessary.

There are no particular restrictions on the concentration of thehydrochloric acid used in the hydrochloric acid washing treatment; theconcentration of common commercially available hydrochloric acid issufficient. Accordingly, a concentration of 0.01 to 37 wt % ispreferable as the concentration of the hydrochloric acid used, and inparticular, if the operating characteristics and safety are taken intoaccount, a concentration of 0.1 to 30 wt % is even more preferable, anda concentration of 0.5 to 25 wt % is especially preferable.

There are no particular restrictions on the temperature at which thehydrochloric acid washing treatment is performed. However, if thistemperature is too high, the hydrochloric acid tends to volatilize, andif this temperature is too low, the heavy metal removal efficiency maydrop. Accordingly, this temperature is preferably in the range of 10 to90° C., and is even more preferably in the range of 20 to 90° C.

Furthermore, there are no particular restrictions on the number of timesthat the hydrochloric acid washing treatment is performed; ordinarily,however, this treatment is performed 1 to 3 times, although this alsodepends on the type and concentration level of the basic substanceremaining in the activated carbon.

It is preferable that the activated carbon that has been subjected tothe above-mentioned hydrochloric acid washing treatment then besubjected to a clean water washing treatment with distilled water or ionexchange water. The activated carbon obtained by the basic washingtreatment may also be immediately subjected to a clean water washingtreatment without being subjected to a hydrochloric acid washingtreatment.

There are no particular restrictions on the amount of distilled water orion exchange water that is used in the clean water washing treatment.However, it is preferable that washing be performed until chlorine ionsor the basic substance can no longer be detected. In concrete terms,although this also depends on the clean water washing system (describedlater), it is ordinarily preferable that the amount of water used be 1to 10,000 parts by weight per parts by weight of activated carbon, andif economy and operating characteristics are taken into account, anamount ranging from 1 to 1000 parts by weight is even more preferable.

Examples of clean water washing systems that can be used include systemsin which washing while agitating is performed using a tank equipped withan agitator, systems in which washing is performed by passing the liquidthrough a filter under reduced pressure or pressurization, or the like.

The activated carbon that is obtained by washing with a liquidcontaining a basic substance, or the activated carbon obtained byfurther performing a hydrochloric acid washing treatment and/or a cleanwater washing treatment if necessary, is dried under heating and/orreduced pressure, thus producing activated carbon in a dry state.

Furthermore, in the activated carbon manufacturing method of the secondinvention of the present application, a carbonization treatment using anordinary method in accordance with the type of carbonaceous materialinvolved (e.g., a dry distillation treatment at 400 to 800° C.) may beperformed prior to the activation treatment if necessary.

In the manufacturing method of the second invention of the presentapplication as well, an easily graphitizable carbonaceous material issubjected to a gas activation treatment or chemical activationtreatment. There are no particular restrictions on the treatment methodused; the gas activation treatment methods or chemical activationtreatment methods described in the first invention of the presentapplication may be used.

For instance, activation treatment methods using steam, carbon dioxide,oxygen, propane combustion waste gas, mixed gases consisting of thesegases or the like may be cited as examples of gas activation treatmentmethods, and activation treatment methods using chemicals such as zincchloride, calcium chloride, phosphoric acid, sulfuric acid, sodiumhydroxide, potassium hydroxide, magnesium hydroxide, calcium hydroxideor the like may be cited as examples of chemical activation treatmentmethods. In particular, in cases where a general alkali activationtreatment using potassium hydroxide or sodium hydroxide as a chemicalagent is performed, alkali metals tend to remain in the activatedcarbon, since an alkali metal hydroxide is used as the activationassistant. Accordingly, in cases where a common alkali activationtreatment is performed, the manufacturing method of the second inventionof the present application can be applied in a preferable manner.

Next, the third invention of the present application will be described.

The activated carbon of the third invention of the present applicationis activated carbon in which the effect of the invention is heightenedby specifying the carbonaceous material in the second invention as aneasily graphitizable carbonaceous and specifying the activationtreatment as an alkali activation treatment, this activated carbon beingobtained by subjecting an easily graphitizable carbonaceous material toan alkali activation treatment. Here, in the activated carbon of thethird invention of the present application, the heavy metal content is20 ppm or less. The reason for this is as follows: specifically, if theheavy metal content exceeds 20 ppm, then in cases where the activatedcarbon is used as a raw material in electronic devices, heavy metalswill be deposited in the electronic devices, leading to problems such asshort-circuiting and the like, and resulting in a drop in a rate ofself-discharge retention. Furthermore, in the activated carbon of thethird invention of the present application, the alkali metal content is200 ppm or less. The reason for this is as follows: specifically, if thealkali metal content exceeds 200 ppm, leakage current is increased andthe charging efficiency drops, so that the energy efficiency is poor,and a rate of self-discharge retention is low.

In the activated carbon of the third invention, nickel, iron, zinc, tin,copper, silver and the like may be cited as examples of heavy metalsthat may be contained in the activated carbon. Among these, heavy metalsthat show large contents are ordinarily one or more of the metalsnickel, iron and zinc, so that it is important to control the contentsof these metals. In particular, it is preferable that the nickel contentbe controlled to 8 ppm or less, that the iron content be controlled to0.3 ppm or less, and that the zinc content be controlled to 0.3 ppm orless. Furthermore, it is preferable that the copper content becontrolled to 1 ppm or less, and that the silver content be controlledto 0.1 ppm or less.

As was described above, the contents of heavy metals in the activatedcarbon can be measured by inductively coupled plasma emissionspectrometry analysis (ICP).

Furthermore, in the third invention of the present application, lithium,sodium, potassium and cesium may be cited as examples of alkali metalsthat may be contained in the activated carbon. Ordinarily, the metalsthat show large contents are sodium and/or potassium so that it isimportant to control the contents of these metals. Furthermore, althoughthere may also be cases in which alkali metals are present as metallicalkalies in the activated carbon, these alkali metals are ordinarilypresent in the form of oxides, hydroxides, chlorides, carbonates or thelike.

As was described above, the contents of alkali metals in the activatedcarbon can be measured by atomic absorption analysis.

In the activated carbon of the third invention of the presentapplication, it is further preferable that the carbon content extractedby means of a hydrocarbon such as benzene, toluene, xylene, mesitylene,a mixture of these hydrocarbons or the like be 0.2 wt % or less. If thiscarbon content exceeds 0.2 wt %, this may lead to pore blockage, a dropin the electrostatic capacitance and a deterioration in the durabilityof the activated carbon, so that such a carbon content is unpreferable.Here, the following series of operations may be cited as an example ofthe extraction operation: specifically, using 5 to 50 parts by weight ofa hydrocarbon per part by weight of activated carbon, these ingredientsare mixed for 1 hour or longer at a temperature exceeding the boilingpoint of the hydrocarbon used, after which the hydrocarbon is filteredout, and the product is dried. The calculation of the carbon contentextracted by means of this hydrocarbon is accomplished by comparing theweights before and after the extraction operation. Furthermore, in theactivated carbon of both the first and second inventions of the presentapplication, it is preferable that the carbon content extracted by meansof the above-mentioned hydrocarbon be 0.2 wt % or less.

Repeated washing with ion exchange water or a strongly acidic aqueoussolution and the like may be cited as means of reducing the overallcontent of heavy metals in the activated carbon of the third inventionof the present application to 20 ppm or less; preferably, however, theactivated carbon manufacturing method of the present invention, which ischaracterized in that washing is performed with an acidic aqueoussolution containing an oxidizing agent (described later), is applied.Furthermore, repeated washing with ion exchange water or a stronglyacidic aqueous solution at room temperature may be cited as means ofreducing the heavy metal content of the activated carbon of the thirdinvention of the present application to 20 ppm or less, and at the sametime reducing the alkali metal content to 200 ppm or less; preferably,however, the activated carbon manufacturing method of the presentinvention, which is characterized in that washing is performed with atleast hot water and hot hydrochloric acid (describe later) is applied.

Examples of carbonaceous materials that can be used in the thirdinvention of the present application include easily graphitizablecarbonaceous materials which form activated carbon when subjected to anactivation treatment, e.g., carbonaceous materials whose starting rawmaterials are petroleum coke, petroleum pitch, synthetic mesophasepitch, polyvinyl chlorides, polyimides, polyacrylonitriles or the like.In particular, mesophase pitch carbon fibers are preferable for use.From the standpoint of superior conductivity, fibers that contain anoptically anisotropic phase at the rate of 50 vol % or greater,preferably 80 vol % or greater, are preferable as mesophase pitch carbonfibers.

Here, mesophase pitch carbon fibers are fibers obtained by subjecting asynthetic mesophase pitch or a mesophase pitch originating frompetroleum or coal to melt spinning, and then subjecting the obtainedmelt-spun fibers to an infusibilizing treatment and a carbonizationtreatment. If the operating characteristics of melt spinning,productivity, operating characteristics during activation andelectrostatic capacitance of the activated carbon obtained are takeninto account, the use of a synthetic mesophase pitch is preferable.Here, the infusibilizing treatment is a treatment which is required inorder to subject the melt-spun pitch fibers to an activation (heat)treatment while maintaining the configuration of the fibers “as is”.Ordinarily, this is a treatment in which melt-spun pitch fibers areheated to a temperature of approximately 100 to 350° C. in an oxidizingatmosphere such as air containing 1 to 20.9 % oxygen or the like. Thecarbonization treatment performed following the infusibilizing treatmentis a treatment in which the product of the infusibilizing treatment iscarbonized by heating this product in an inert gas atmosphere. If theheating temperature is too low, over-activation will occur due toincomplete development of the carbon crystal structure, so that thedensity tends to show an excessive drop; on the other hand, if thistemperature is too high, crystallization of the carbon fibers willprogress to an excessive extent, so that activation tends not toproceed. Accordingly, the heating temperature is preferably 500 to 1000°C., and is even more preferably 600 to 900° C.

In the third invention of the present application, the carbonaceousmaterial is preferably used in a pulverized state. In order to ensuregood mixing with the alkali metal hydroxide used as an activationassistant, so that the alkali activation treatment (described later) iscaused to proceed in an effective manner, it is preferable to use apulverized material in which the maximum length in the direction of thelong axis is 500 μm or less, preferably 200 μm or less. For example, themaximum length in the direction of the long axis can be confirmed byobserving electron micrographs of randomly extracted samples of thepulverized carbonaceous material. Pulverization can be accomplishedusing a universally known pulverizer such as a cone crusher, double rollcrusher, disk crusher, rotary crusher, ball mill, centrifugal rollermill, ring roll mill, centrifugal ball mill, turbo mill or the like.

A conventional universally known alkali activation treatment using analkali metal hydroxide as an activation assistant may be cited as anexample of the alkali activation treatment used in the third inventionof the present application.

The activated carbon of the third invention of the present applicationdescribed above can be manufactured in a preferable manner by theactivated carbon manufacturing method of the third invention of thepresent application, which will be described below.

Specifically, the activated carbon manufacturing method of the thirdinvention of the present application is a method in which activatedcarbon is obtained by subjecting a easily graphitizable to an alkaliactivation treatment, and then washing the activation treatment productthus obtained with an acidic aqueous solution containing an oxidizingagent. As a result of this washing using an acidic aqueous solutioncontaining an oxidizing agent, the heavy metals contained in theactivation treatment product migrate to the side of the cleaning liquid.Accordingly, the content of heavy metals remaining in the activatedcarbon can be reduced to 20 ppm or less. The reason for this is unclear;however, it is thought that the heavy metals contained in the activationtreatment product are cleaned and removed because by washing theactivation treatment product with an acidic aqueous solution containingan oxidizing agent, these metals become ionic compounds with good watersolubility.

In the manufacturing method of the third invention of the presentapplication, an easily graphitizable carbonaceous material is firstsubjected to an alkali activation treatment, so that an activationtreatment product is obtained. Here, the alkali activation treatment isa treatment in which an easily graphitizable carbonaceous material ismixed by ordinary methods with an alkali metal hydroxide constituting anactivation assistance, and the mixture thus obtained is activated byheating in an inert gas atmosphere such as nitrogen gas or the like.

Examples of alkali metal hydroxides that can be used as activationassistants include sodium hydroxide, potassium hydroxide, lithiumhydroxide and the like. Among these, it is preferable to use sodiumhydroxide or potassium hydroxide in order to obtain activated carbonthat shows a large electrostatic capacitance. These alkali metalhydroxides may be used singly or in mixtures. Furthermore, the alkalimetal hydroxides may also be used in the form of a powder or in the formof a concentrated aqueous solution.

In regard to the amount of alkali metal hydroxide that is used relativeto the easily graphitizable carbonaceous material, if this amount is toosmall, it is difficult to perform a sufficient and uniform activationtreatment, so that some variation may occur in the properties of thedesired activated carbon. Conversely, if this amount is too large, theprocess becomes uneconomical, and there is a danger that activation willproceed to an excessive extent, so that while the electrostatic capacityper unit weight tends to increase, a drop in the electrostatic capacityper unit volume is generated. Accordingly, taking economy, operatingcharacteristics and safety into account, the amount of alkali metalhydroxide used relative to the carbonaceous material is preferably 30 to500 parts by weight, and even more preferably 50 to 300 parts by weight,per 100 parts by weight of the carbonaceous material.

In regard to the heating temperature conditions used to activate theeasily graphitizable carbonaceous raw that is mixed with theabove-mentioned alkali metal hydroxide, if the heating temperature istoo low, the activation will be insufficient. On the other hand, if thetemperature is too high, the crystallization of the activated carbonthat is obtained will progress so that the electrostatic capacitancedrops when the activated carbon is used as an electrode in an electricaldouble layer capacitor; moreover, the danger caused by alkali metalsgenerated from the alkali metal hydroxide used is increased, and even ifa material based on nickel, which has a high resistance to alkalimetals, is used in the activation furnace, grain boundary corrosionaccompanying crystallization of the metal material (furnace material)will be accelerated, so that the admixture of heavy metal particles inthe activated carbon is conspicuously increased. Accordingly, atemperature of 400° C. to 1000° C. is preferable, and a temperature of450° C. to 900° C. is even more preferable, as a heating temperature foraccomplishing activation (activation temperature); furthermore, ifeconomy related to the quantity of heat is taken into account, atemperature of 470° C. to 850° C. is especially preferable.

Taking the problem of grain boundary crystallization of theabovementioned activation vessel into account, the rate of temperatureelevation to the activation temperature is preferably 0.2° C. to 10°C./minute, and is even more preferably 0.3 to 8° C./min, in order toavoid an abrupt temperature elevation.

There are no particular restrictions on the retention time at theactivation temperature, as long as this is a time that allows thetransfer of a sufficient quantity of heat to the activated carbon;ordinarily, this time is 30 minutes to 5 hours, and if the progressionof crystallization along with this retention is taken into account, thetime is preferably 45 minutes to 4 hours.

The rate of cooling from the activation temperature is important fromthe standpoint of avoiding grain boundary crystallization of theactivation furnace body. Specifically, since rapid cooling promotesmetal crystallization, the cooling rate is preferably 1 to 50°C./minute, and is even more preferably 1 to 30° C./minute.

Next, in the manufacturing method of the third invention of the presentapplication, the activation treatment product obtained by subjecting theabove-mentioned easily graphitizable carbonaceous to an alkaliactivation treatment is washed with an “acidic aqueous solutioncontaining an oxidizing agent” (oxidizing agent washing treatment). As aresult of the activation treatment produced being subjected to anoxidizing agent washing treatment, heavy metals admixed from the furnacebody subjected to grain boundary corrosion as a result of being oxidizedduring the alkali activation treatment by the use of an alkali metalhydroxide can be effectively removed from the activation treatmentproduct.

Furthermore, it is also preferable that the activation treatment productbe washed beforehand with warm water prior to the oxidizing agentwashing treatment. As a result of this washing, alkali metal hydroxides,alkali metal carbonates and the like remaining in the activationtreatment product can be removed. Specifically, by maximizing theremoval of substances that can react with an acidic aqueous solutionfrom the activation treatment product by washing with warm water whenthe oxidizing agent washing treatment is performed, it is possible toreduce that amount of the “acidic aqueous solution containing anoxidizing agent” that is used. There are no restrictions on thetemperature or amount of warm water that is used; ordinarily, however,the temperature is 30° C. to 90° C., and the amount used is preferably 3to 50 parts by weight per part by weight of the activation treatmentproduct that is to be washed. Taking the washing efficiency andvolumetric efficiency of the reaction vessel into account, the amountused is even more preferably 5 to 45 parts by weight.

In the manufacturing method of the third invention of the presentapplication, examples of oxidizing agents that can be used in the“acidic aqueous solution containing an oxidizing agent” includeinorganic oxidizing agents such as hydrogen peroxide, persulfuric acidand the like, organic per-acids such as peracetic acid, performic acidand the like, and organic peroxides such as t-butyl hydroperoxide andthe like. In particular, taking into account stability in the acidicaqueous solution, availability, safety and effect on the carbonmaterial, hydrogen peroxide is most preferable.

The concentration of the oxidizing agent in such an “acidic aqueoussolution containing an oxidizing agent” is preferably 0.1 to 10 wt %. Ifeconomy and effect on the carbon material are taken into account, theconcentration is more preferably 0.2 to 5 wt %, and is most preferably0.5 to 2 wt %.

Examples of acid aqueous solutions that can be used as the “acidicaqueous solution containing an oxidizing agent”include aqueous solutionsof organic acids such as formic acid, acetic acid and the like, andaqueous solutions of inorganic acids such as hydrochloric acid, sulfuricacid, nitric acid and the like. Among these acids, inorganic acids suchas hydrochloric acid, sulfuric acid, nitric acid and the like arepreferable from the standpoint of the effect in reducing the residualamounts of metals; furthermore, if presence of acidic radicals on thecarbonaceous material is taken into account, the use of hydrochloricacid is most preferable. Furthermore, the concentration of the acid ispreferably 0.1 to 5 N; if the ability to remove metals and the effectson the carbon material are taken into account, a concentration of 0.2 to4 N is even more preferable, and if economy and operatingcharacteristics are taken into account, a concentration of 0.5 to 3 N isespecially preferable.

The amount of the “acidic aqueous solution containing an oxidizingagent” that is used relative to the activation treatment product ispreferably 3 to 50 parts by weight per part by weight of the activationtreatment product that is to be washed. If operating characteristics andeconomy are taken into account, this amount is even more preferably 4 to30 parts by weight, and is most preferably 5 to 20 parts by weight.

There are no particular restrictions on the washing temperature duringthe oxidizing agent washing treatment. However, if this temperature istoo high, this will lead to the decomposition of the peroxideconstituting the oxidizing agent, and oxidation of the carbon materialmay proceed. On the other hand, if this temperature is too low, theeffect of the oxidizing agent tends to drop. Accordingly, thistemperature is preferably 0° C. to 50° C., and is even more preferably0° C. to 45° C.

There are no particular restrictions on the number of times that theoxidizing agent washing treatment is performed; this number of timesalso depends on the amounts of metals remaining in the activationtreatment product and the desired residual metal amount levels in theactivated carbon, and is ordinarily in the range of 1 to 3 times.

There are no particular restrictions on the pressure that is appliedwhen the activation treatment product is washed with the “acidic aqueoussolution containing an oxidizing agent” in the oxidizing agent washingtreatment. However, if this pressure is too high, a special apparatus isrequired; ordinarily, therefore, this pressure is a pressure rangingfrom atmospheric pressure to 1 MPa, and is preferably a pressure rangingfrom atmospheric pressure to 0.5 MPa.

Activated carbon is obtained by performing an oxidizing agent washingtreatment on the activation treatment product; however, it is preferablethat the activated carbon that is obtained be further washed withdistilled water or ion exchange water (clean water washing treatment).There are no particular restrictions on the amount of distilled water orion exchange water that is used in the clean water washing treatment;however, it is preferable to wash this product until chlorine ions canno longer be detected. Although this amount also depends on the washingsystem used, it is ordinarily preferable that the amount used be in therange of 1 part by weight to 10,000 parts by weight per part by weightof activated carbon; if economy and operating characteristics are takeninto account, the amount is even more preferably 1 part by weight to1000 parts by weight per part by weight of activated carbon.

Examples of clean water washing systems that can be used include systemsin which washing while agitating performed using a tank equipped with anagitator, systems in which washing is performed by passing the liquidthrough a filter under reduced pressure or pressurization, or the like.

The activated carbon that is obtained by washing with theabove-mentioned “acidic aqueous solution containing an oxidizing agent”,or the activated carbon that is obtained by a further hydrochloric acidwashing treatment and/or clean water washing treatment if necessary, isdried under heating and/or a reduced pressure, so that activated carbonin a dry state is produced.

Furthermore, this third invention of the present application provides amanufacturing method in which a easily graphitizable carbonaceousmaterial to an alkali activation treatment, and the activation treatmentproduct thus obtained is (1) washed with hot water, hot hydrochloricacid and water in that order, (2) washed with hot water, carbonatedwater, hot hydrochloric acid, aqueous ammonia and hot water in thatorder, or (3) washed with hot water, carbonated water, hot hydrochloricacid, aqueous ammonia, hot hydrochloric acid and hot water in thatorder, as a manufacturing method which is suitable for manufacturingactivated carbon in which not only the overall content of heavy metalsis 20 ppm or less, but also the overall content of alkali metals is 200ppm or less.

Here, the solubility of alkali metal compounds such as alkali metalhydroxides, alkali metal carbonates and the like in hot water is greatlyimproved compared to the solubility of such compounds in cold water.Furthermore, the reactivity of heavy metals with hot hydrochloric acidis greatly improved compared to the reactivity of such metals withhydrochloric acid at room temperature. Accordingly, by washing theactivation treatment product with hot hydrochloric acid, it is possibleto convert the heavy metals contained in the activation treatmentproduct into chlorides that are readily soluble in water. Consequently,the content of heavy metals remaining in the activated carbon can bereduced to 20 ppm or less, and the content of alkali metals can bereduced to 200 ppm or less, by washing the activation treatment productwith hot water, and then washing this produced with hot hydrochloricacid. Furthermore, in the manufacturing method of the present invention,it is sufficient to arrange the system so that the water andhydrochloric acid used for washing are heated; accordingly, activatedcarbon can be manufactured at a high yield without greatly modifyingconventional alkali activation treatment equipment.

In the abovementioned procedure of (1) in the manufacturing method ofthe third invention of the present application, the activation treatmentproduct obtained by the alkali activation treatment of a carbonaceousmaterial is first washed with hot water (hot water washing), and is thenwashed with hot hydrochloric acid (hot hydrochloric acid washing),followed by a washing with water (water washing). Here, alkali metalhydroxides, alkali metal carbonates and the like remaining in theactivation treatment product can be efficiently removed by washing withhot water. Furthermore, the hot water washing makes it possible toremove substances capable of reacting with the acidic aqueous solutionin the subsequent hot hydrochloric acid washing, so that the effect ofthe hot hydrochloric acid can be heightened, and so that the amount ofacidic aqueous solution that is used can be reduced. There are norestrictions on the temperature and amount of hot water used; thetemperature is preferably 30° C. to 95° C., and is even more preferably60 to 90° C., while the amount of hot water used is preferably 3 to 50parts by weight per part by weight of the activation treatment productthat is to be washed, and is even more preferably 5 to 45 parts byweight if the washing efficiency and volumetric efficiency of thereaction vessel are taken into account.

Next, the activation treatment product that has been washed with hotwater is washed with hot hydrochloric acid. In regard to theconcentration of the hydrochloric acid, if this concentration is toolow, this may lead to passive states of the heavy metals so that theremoval efficiency drops; on the other hand, if this concentration istoo high, the treated activated carbon is chlorinated. Accordingly, itis preferable to perform this treatment in a concentration range of 0.1to 3 N, preferably 0.5 to 2.5 N. Furthermore, in regard to thetemperature of hot hydrochloric acid that is used, if this temperatureis too low, the ability to remove partially oxidized heavy metals drops,and the passive states of heavy metals in hydroxide or carbonate form ispromoted, so that the metal removal efficiency may drop. On the otherhand, if this temperature is too high, the volatilization ofhydrochloric acid becomes severe, so that the efficiency of thehydrochloric acid drops, and so that the problem of corrosion of theequipment used also becomes severe. Accordingly, a temperature range of60° C. to 90° C. is appropriate for the use temperature of the hothydrochloric acid. The amount of hot hydrochloric acid used ispreferably 3 to 50 parts by weight per part by weight of the activationtreatment product that is to be washed; if operating characteristics andeconomy are taken into account, an amount of 4 to 30 parts by weight iseven more preferable, and an amount of 5 to 20 parts by weight is mostpreferable.

Next, the activation treatment product that has been washed with hotwater and washed with hot hydrochloric acid is washed with water. As aresult, activated carbon is obtained. Since this activated carbon isaffected by residual metals contained in the water used, it ispreferable that clean water such as ion exchange water, distilled water,water that has passed through a membrane or the like be used as the washwater. There are no particular restrictions on the amount of wash waterused; however, it is preferable that washing be performed until chlorineions can no longer be detected. Although this amount also depends on thewashing system used, the amount used is ordinarily in the range of 1part by weight to 10,000 parts by weight per part by weight of activatedcarbon, and if economy and operating characteristics are taken intoaccount, it is preferable to use an amount in the range of 1 part byweight to 1000 parts by weight per part by weight of activated carbon.

Examples of clean water washing systems that can be used include systemsin which washing while agitating is performed using a tank equipped withan agitator, systems in which washing is performed by and passing theliquid through using a filter under reduced pressure or pressurization,or the like.

In the manufacturing method of the third invention of the presentapplication, there are no particular restrictions on the pressure thatis applied when the hot water washing, hot hydrochloric acid washing andwater washing are performed. However, if this pressure is too high, aspecial apparatus is required. Ordinarily, therefore, this pressure is apressure in the range of atmospheric pressure to 1 MPa, and ispreferably a pressure in the range of atmospheric pressure to 0.5 MPa.

Furthermore, in the abovementioned procedure of (2) in the manufacturingmethod of the third invention of the present application, the activationtreatment product obtained by subjecting a carbonaceous material to analkali activation treatment is first washed with hot water (hot waterwashing), and is then washed with carbonated water (carbonated waterwashing), washed with hot hydrochloric acid (hot hydrochloric acidwashing), washed with aqueous ammonia (aqueous ammonia washing), andwashed with hot water (hot water washing). The points of difference fromthe procedure of (1) are the abovementioned washing with carbonatedwater, washing with aqueous ammonia, and final washing with hot water.Here, the carbonated water washing operation is an operation that isperformed in order to remove the alkali metals in the activationtreatment product as carbonates, and is performed in the same manner asin the case described in the manufacturing method of the first inventionof the present application.

Furthermore, the aqueous ammonia washing operation is an operation whichis used to remove heavy metals in the activation treatment product asammonia complexes, and is performed in the same manner as in the casedescribed in the manufacturing method of the second invention of thepresent application.

Moreover, the final hot water washing operation can be performed in thesame manner as the hot water washing operation of the above-mentionedprocedure of (1).

Furthermore, in the abovementioned procedure of (3) in the manufacturingmethod of the third invention of the present application, the activationtreatment product that has been obtained by subjecting a carbonaceousmaterial to an alkali activation treatment is first washed with hotwater (hot water washing), and is then washed with carbonated water(carbonated water washing), washed with hot hydrochloric acid (hothydrochloric acid washing), washed with aqueous ammonia (aqueous ammoniawashing), again washed with hot hydrochloric acid, and washed with hotwater (hot water washing). The point of difference from the procedure of(2) is that hot hydrochloric acid washing is again performed followingwashing with aqueous ammonia.

This hot hydrochloric acid washing is as was described above;ammonia-derived substances (residues, metal complexes) in the activationtreatment product can be removed by this washing operation.

The activated carbon obtained by the manufacturing method of the thirdinventions of the present application described above is dried underheating and/or a reduced pressure, thus producing activated carbon in adry state.

The activated carbon of the first, second and third inventions of thepresent application described above is especially useful as a materialfor polarizing electrodes used in electrical double layer capacitors.Accordingly, the fourth invention of the present application provides apolarizing electrode which is formed by mixing the activated carbon ofthe first, second or third invention of the present application with atleast a binder and a conductive material, and an electrical double layercapacitor using this electrode.

The polarizing electrode of the fourth invention of the presentapplication will be described below.

The polarizing electrode of the fourth invention of the presentapplication is formed by mixing at least a binder such as apolyvinylidene fluoride, polytetrafluoroethylene or the like and aconductive material such as carbon black or the like with the activatedcarbon of the first, second or third invention of the presentapplication, and molding this mixture. The resistance of the electrodecan be lowered by mixing a conductive material; as a result, this iseffective in lowering the internal resistance of the polarizingelectrode.

Ordinary known methods can be used in order to manufacture such apolarizing electrode suitable for use in an electrical double layercapacitor. For example, a substance known as a binder, such as acommercially available marketed polyvinylidene fluoride,polytetrafluoroethylene or the like, and a conductive material such ascarbon black or the like can be added, as necessary, in amounts up to afew percent, and then thoroughly kneaded. Afterward, this mixture can bemolded into an electrode by press-molding in a metal mold or rollinginto the form of a sheet, and punching the resulting body into therequired shape. Alternatively, the kneaded mixture may be applied as acoating to a current collector, and formed into a coating electrode.Solvents such as water, organic compounds such as alcohol,N-methylpyrrolidone and the like, dispersing agents and various types ofadditives may be used as necessary in this electrode formation process.Furthermore, heat may be applied within limits that cause no loss of theeffect of the invention.

The polarizing electrode described above is useful as an electrode inthe electrical double layer capacitor of the fourth invention of thepresent application, which is shown in FIG. 1 (schematic sectionalview). The respective constituent elements that constitute the capacitorshown in FIG. 1 can be constructed in the same manner as in auniversally known electrical double layer capacitor, except for the factthat the polarizing electrode of the present invention is used. Forexample, in the figure, 1 and 2 indicate current collecting memberscomprised of aluminum or the like, 3 and 4 indicate polarizingelectrodes consisting of the activated carbon of the present invention,5 indicates a separator constructed from a polypropylene non-wovenfabric or the like, 6 indicates a gasket constructed from apolypropylene, polyethylene, polyamide, polyamidoimide, polybutylene orthe like, and 7 indicates a case constructed from a material such asstainless steel or the like.

Furthermore, in order to cause this device to function as an electricaldouble layer capacitor, an electrolyte solution prepared by dissolving auniversally known electrolyte such as tetraethylammoniumtetrafluoroborate, tetramethylammonium tetrafluoroborate or the like ina solvent, e.g., a carbonate such as ethylene carbonate, dimethylcarbonate, diethyl carbonate, propylene carbonate or the like, a nitrilesuch as acetonitrile or the like, a lactone such as γ-butyrolactone,α-methyl-γ-butyrolactone or the like, a sulfoxide such as dimethylsulfoxide or the like, or an amide such as dimethylformamide or thelike, must be sealed inside the case 7.

Since the electrical double layer capacitor of the construction shown inFIG. 1 uses the activated carbon of the present invention, thiscapacitor shows the rate of high self-discharge retention.

EXAMPLES

The present invention will be described more concretely below byreferring to the Examples.

Example A1

An optically anisotropic pitch with a Mettler softening point of 285° C.obtained by heat-treating a petroleum decomposition residue was spun bymelt blow-spinning using a nozzle which had 1000 spinning holes with adiameter of 0.2 mm in a slit having a width of 2 mm, and these spunfibers were subjected to an infusibilizing treatment and a carbonizationtreatment, thus producing mesophase pitch carbon fibers.

50 g of mesophase pitch fibers that were pulverized so that the maximumlength of the fibers in the direction of the long axis was 20 μm orless, and 10 g of 95% potassium hydroxide that was pulverized to a meanparticle size of 1 mm or less were places in a 300-mL (milliliter) glassseparable flask in which a thermometer and agitator were mounted, andthis mixture was agitated at 10 rpm while nitrogen was caused to flowthrough at a rate of 200 mL/minute. This separable flask was heated bymeans of an oil bath, and the mixture was agitated for 1 hour at aninternal temperature of 160° C. Afterward, the heat source was removed,and the mixture was agitated for an additional hour while nitrogen waspassed through the system, thus producing a granular substance. Thegranular substance had a size of 20 mm or less. Next, the granularsubstance was dehydrated under a reduced pressure of 1.5 Torr, with thetemperature elevated to 300° C. over a period of 5 hours at atemperature elevation rate of 2° C./minute.

24 g of the dehydrated granular substance thus obtained was placed in a2-inch horizontal nickel reaction vessel in which a thermometer wasmounted, and after the air inside the system was replaced with nitrogen,the temperature was elevated to 700° C. at a rate of 200° C./hour undera nitrogen current of 100 mL/minute. After the temperature reached 700°C., this temperature was maintained for 1 hour; afterward, the systemwas cooled to room temperature over a period of 2 hours. Nitrogenpassing through a distilled water bubbler was passed through the systemor 1 hour, and the reaction mixture was then placed in 150 mL of water.

After the supernatant was removed by decantation, 150 mL of water wasagain added, and the system was agitated. The supernatant was removed bydecantation; then, 100 mL of carbonated water (carbonic acidconcentration 1 wt %) was added at 10° C. and the system was agitated,after which the supernatant was removed by decantation. This operationwas performed twice; afterward, 200 mL of a 1 N aqueous solution ofhydrochloric acid was added, and the system was neutralized and washed.Then, the system was continuously washed using 3 L of distilled water sothat alkali metal salts were removed. The system was then dried toproduce 6.7 g of activated carbon.

Comparative Example A1

Activated carbon was obtained by repeating the same operation as inExample A1, except that washing with carbonated water was not performed.

Example A2

5.9 g of activated carbon was obtained by repeating the same operationas in Example A1, except that the carbon material used was 50 g of acarbonized phenolic resin pulverized to 20 μm or less.

Comparative Example A2

Activated carbon was obtained by repeating the same operation as inExample A2, except that washing with carbonated water was not performed.

(Evaluation)

100-mg samples of the respective activated carbon products obtained inExamples A1 and A2 and Comparative Examples A1 and A2 were subjected towet decomposition using 200 mL of nitric acid, and then 20 mL ofperchloric acid. Afterward, the residual potassium and sodium metalcomponents were measured using an atomic absorption analysis device(Deflecting Zeeman atomic absorption photometer Z-5300, manufactured byHitachi Seisakusho K.K.). The results obtained are shown in Table 1.Furthermore, measurements were also performed for other alkali metalcomponents (lithium, cesium); however, the contents of these componentswere below the detection limit (1 ppm). Accordingly, the overall alkalimetal content was substantially equal to the total-content of potassiumand sodium.

Furthermore, the respective activated carbon products obtained inExamples A1 and A2 and Comparative Examples A1 and A2 were furtherpulverized to a mean particle size of 5 to 20 μm, thus producingpowdered activated carbon samples, and mixtures consisting of 80 wt % ofpowdered activated carbon, 10 wt % of conductive carbon and 10 wt % ofpolytetrafluoroethylene (Teflon (registered trademark) 6J, Mitsui-duPont Chemical Col.) were kneaded. The kneaded mixtures thus obtainedwere molded into sheets with a thickness of 300 μm by rolling, and werepunched into circular shapes with a diameter of 2 cm using a punchingdevice. Afterward, sheet-form polarizing electrodes were manufactured bydrying these punched sheets for 4 hours at 150° C. under reducedpressure.

Using the polarizing electrodes thus obtained, a current collectingmember, polarizing electrode sheet, polypropylene non-woven fabric,polarizing electrode sheet and current collecting member were laminatedin this order in a stainless steel case (as shown in FIG. 1) inside aglove box with a dew point of −80° C. or less. Afterward, the polarizingelectrodes were impregnated with a propylene carbonate solutioncontaining 1 mole of tetraethylammonium tetrafluoroborate, and weresealed by crimping to the upper lid of the stainless steel case using aninsulating gasket made of polypropylene, thus producing electricaldouble layer capacitors.

The electrical double layer capacitors thus obtained were charged with aconstant current at 2 mA/cm relative to the electrode surface area up toan attained voltage of 2.5 V at room temperature using an electricaldouble layer capacitor evaluation device manufactured by HIOKI E.E.CORPRATION, and were then subjected to supplementary charging at a lowvoltage for 30 minutes at 2.5 V. Following the completion ofsupplementary charging, a charge-discharge cycle test in which aconstant-current discharge was performed at 2 mA/cm was repeated 10times, and the initial electrostatic capacitance was determined by aconventional method on the basis of the discharge curve from 1.2 V to1.0 V in this case. The results obtained are shown in Table 1.

The electrical double layer capacitors whose electrostatic capacitancehad thus been determined were charged with a constant current at 2mA/cm² relative to the electrode surface area up to an attained voltageof 2.5 V at room temperature, and were then subjected to supplementarycharging at a low voltage for 30 minutes at 2.5 V. Following thecompletion of supplementary charging, the capacitors were allowed tostand for 50 hours; afterward, a constant-current discharge wasperformed at 2 mA/cm², and the electrostatic capacitance after thecapacitors were allowed to stand was determined by a conventional methodon the basis of the discharge curve form 1.2 V to 1.0 V in this case.The rate of self-discharge retention (%) was determined by dividing thisvalue by the previously determined initial electrostatic capacitance.The results obtained are shown in Table 1. TABLE 1 Comparative ExampleExample A1 A2 A1 A2 Na content (ppm) 6 2 3 4 K content (ppm) 54 84 226156 Overall alkali metal 60 86 229 160 content (ppm) Initialelectrostatic 28.5 17.0 28.3 17.1 capacitance (F/cc) Electrostaticcapacitance 27.9 16.8 25.1 15.3 after standing (F/cc) Rate ofself-discharge 97.8 98.8 88.0 89.4 retention (%)

As is seen from Table 1, the electrical double layer capacitorsmanufactured from the activated carbon of Example A1 and Example A2, inwhich the overall alkali metal content was 100 ppm or less, showed therates of self-discharge retention close to 100%, and thus showedsuperior performance as electrical double layer capacitors.

On the other hand, the electrical double layer capacitors manufacturedfrom the activated carbon of Comparative Example A1 and ComparativeExample A2, in which the overall alkali metal content exceeded 100 ppm,showed the rate of self-discharge retention of less than 90%, and thusdid not show sufficient performance as electrical double layercapacitors.

Example B1

An optically anisotropic pitch with a Mettler softening point of 285° C.obtained by heat-treating a petroleum decomposition residue was spun bymelt blow-spinning using a nozzle which had 1000 spinning holes with adiameter of 0.2 mm in a slit having a width of 2 mm, and these spunfibers were subjected to an infusibilizing treatment and a carbonizationtreatment, thus producing mesophase pitch carbon fibers.

50 g of mesophase pitch fibers that were pulverized so that the maximumlength of the fibers in the direction of the long axis was 20 μm orless, and 100 g of 95% potassium hydroxide that was pulverized to a meanparticle size of 1 mm or less were places in a 300-mL (milliliter) glassseparable flask in which a thermometer and agitator were mounted, andthis mixture was agitated at 10 rpm while nitrogen was caused to flowthrough at a rate of 200 mL/minute. This separable flask was heated bymeans of an oil bath, and the mixture was agitated for 1 hour at aninternal temperature of 160° C. Afterward, the heat source was removed,and the mixture was agitated for an additional hour while nitrogen waspassed through the system, thus producing a granular substance. Thegranular substance had a size of 20 mm or less. Next, the granularsubstance was dehydrated under a reduced pressure of 1.5 Torr, with thetemperature elevated to 300° C. over a period of 5 hours at atemperature elevation rate of 2° C./minute.

24 g of the dehydrated granular substance thus obtained was placed in a2-inch horizontal nickel reaction vessel in which a thermometer wasmounted, and after the air inside the system was replaced with nitrogen,the temperature was elevated to 700° C. at a rate of 200° C./hour undera nitrogen current of 100 mL/minute. After the temperature reached 700°C., this temperature was maintained for 1 hour; afterward, the systemwas cooled to room temperature over a period of 2 hours. Nitrogenpassing through a distilled water bubbler was passed through the systemor 1 hour, and the reaction mixture was then placed in 150 mL of water.

After the supernatant was removed by decantation, 150 mL of water wasagain added, and the system was agitated. The supernatant was removed bydecantation; then, 100 mL of 5% aqueous ammonia was added at 30° C. andthe system was agitated, after which the supernatant was removed bydecantation. This operation was performed twice; afterward, 200 mL of a1 N aqueous solution of hydrochloric acid was added, and the system wasneutralized and washed. Then, the system was continuously washed using 3L of distilled water so that salts were removed. The system was thendried to produce 6.4 g of activated carbon.

Comparative Example B1

Activated carbon was obtained by repeating the same operation as inExample B1, except that washing with aqueous ammonia was not performed.

Example B2

5.9 g of activated carbon was obtained by repeating the same operationas in Example B1, except that 50 g of a carbonized phenolic resinpulverized to 20 μm or less was used as the carbon material in ExampleB1.

Comparative Example B2

Activated carbon was obtained by repeating the same operation as inExample B2, except that washing with aqueous ammonia was not performed.

(Evaluation)

100-mg samples of the respective activated carbon products obtained inExamples B1 and B2 and Comparative Examples B1 and B2 were subjected towet decomposition using 200 mL of nitric acid, and then 20 mL ofperchloric acid. Afterward, the contents of residual heavy metalsincluding nickel, copper, zinc and iron were measured using aninductively coupled plasma emission spectrometry analysis device (ICPanalysis device, IRIS AP manufactured by Thermo Electron Corporation).The results obtained are shown in Table 2.

Furthermore, the respective activated carbon products obtained inExamples B1 and B2 and Comparative Examples B1 and B2 were furtherpulverized to a mean particle size of 5 to 20 μm, thus producingpowdered activated carbon samples, and mixtures consisting of 80 wt %powdered activated carbon, 10 wt % conductive carbon and 10 wt %polytetrafluoroethylene (Teflon (registered trademark) 6J, Mitsui-duPont Chemical Col.) were kneaded. The kneaded mixtures thus obtainedwere molded into sheets with a thickness of 300 μm by rolling, and werepunched into circular shapes with a diameter of 2 cm using a punchingdevice. Afterward, sheet-form polarizing electrodes were manufactured bydrying these punched sheets for 4 hours at 150° C. under reducedpressure.

Using the polarizing electrodes thus obtained, a current collectingmember, polarizing electrode sheet, polypropylene non-woven fabric,polarizing electrode sheet and current collecting member were laminatedin this order in a stainless steel case (as shown in FIG. 1) inside aglove box with a dew point of −80° C. or less. Afterward, the polarizingelectrodes were impregnated with a propylene carbonate solutioncontaining 1 mole of tetraethylammonium tetrafluoroborate, and weresealed by crimping to the upper lid of the stainless steel case using aninsulating gasket made of polypropylene, thus producing electricaldouble layer capacitors.

The electrical double layer capacitors thus obtained were charged with aconstant current at 2 mA/cm² relative to the electrode surface area upto an attained voltage of 2.5 V at room temperature using an electricaldouble layer capacitor evaluation device manufactured by HIOKI E.E.CORPORATION, and were then subjected to supplementary charging-at a lowvoltage for 30 minutes at 2.5 V. Following the completion ofsupplementary charging, a charge-discharge cycle test in which aconstant-current discharge was performed at 2 mA/cm² was repeated 10times, and the initial electrostatic capacitance was determined by aconventional method on the basis of the discharge curve from 1.2 V to1.0 V in this case. The results obtained are shown in Table 2.

The electrical double layer capacitors whose electrostatic capacitancehad thus been determined were charged with a constant current at 2mA/cm2 relative to the electrode surface area up to an attained voltageof 2.5 V at room temperature, and were then subjected to supplementarycharging at a low voltage for 30 minutes at 2.5 V. Following thecompletion of supplementary charging, the capacitors were allowed tostand for 50 hours; afterward, a constant-current discharge wasperformed at 2 mA/cm², and the electrostatic capacitance after thecapacitors were allowed to stand was determined by a conventional methodon the basis of the discharge curve form 1.2 V to 1.0 V in this case.The rate of self-discharge retention (%) was determined by dividing thisvalue by the previously determined initial electrostatic capacitance.The results obtained are shown in Table 2. TABLE 2 Comparative ExampleExample B1 B2 B1 B2 Ni content (ppm) 6 27 4 31 Cu content (ppm) 0.1 0.31.1 1.04 Zn content (ppm) 0.05 0.07 0.09 0.12 Fe content (ppm) 0.12 0.180.41 0.47 Overall heavy metal content 8.5 4.9 30.1 33.2 (ppm) Initialelectrostatic 33.2 31.5 23.5 23.4 capacitance (F/cc) Electrostaticcapacitance 30.4 27.7 22.1 18.4 after standing (F/cc) Rate ofself-discharge 97.4 91.1 87.9 78.6 retention (%)

As is seen from Table 2, the electrical double layer capacitorsmanufactured from the activated carbon of Example B1 and Example B2, inwhich the overall heavy metal content was 20 ppm or less, showed therate of self-discharge retention of 90% or greater, and thus showedsuperior performance as electrical double layer capacitors.

On the other hand, the electrical double layer capacitors manufacturedfrom the activated carbon of Comparative Example B1 and ComparativeExample B2, in which the overall heavy metal content exceeded 20 ppm,showed the rates of self-discharge retention of less than 90%, and thusdid not show sufficient performance as electrical double layercapacitors.

Example C1

An optically anisotropic pitch with a Mettler softening point of 285° C.obtained by heat-treating a petroleum decomposition residue was spun bymelt blow-spinning using a nozzle which had 1000 spinning holes with adiameter of 0.2 mm in a slit having a width of 2 mm, and these spunfibers were subjected to an infusibilizing treatment and a carbonizationtreatment, thus producing mesophase pitch carbon fibers.

The mesophase pitch carbon fibers thus obtained were pulverized to 0.02mm, and 200 g of 95% potassium hydroxide was added and mixed with 100 gof the pulverized fibers thus obtained. This mixture was placed in anickel reaction tube with a diameter of 4 inches in which a nitrogenintroduction tube and a off-gassing line were mounted. This reactiontube was set in a cylindrical furnace, and the temperature was elevatedto 700° C. at a temperature elevation rate of 3.3° C./minute whilenitrogen was caused to flow through at a rate of 100 milliliters(mL)/minute. The system was maintained at this temperature for 1 hour.Afterward, the system was cooled to room temperature at a rate of 5°C./minute, thus producing an activation treatment product.

The activation treatment product thus obtained was placed in apressurized filter with a diameter of 3 cm, and was washed at a pressureof 0.2 MPa using 2 liters (L) of ion exchange water at 60° C. Next, thisproduct was washed at a temperature of 40° C. and a pressure of 0.2 MPausing 2 L of 1 N hydrochloric acid containing 1 wt % hydrogen peroxide.Furthermore, this product was then washed at a temperature of 60° C. anda pressure of 0.2 MPa using 4 L of ion exchange water, thus producingactivated carbon. The activated carbon thus obtained was heated for 3hours at 100° C., and was then heated for 8 hours at a pressure of 0.1MPa and dried under reduced pressure, thus producing activated carbon ina dry state.

Example C2

Activated carbon was manufactured by the same operation as in ExampleC1, except that 1 N hydrochloric acid containing 2 wt % hydrogenperoxide was used as the acidic aqueous solution.

Comparative Example C1

Activated carbon was manufactured by the same operation as in ExampleC1, except that hydrochloric acid containing no hydrogen peroxide wasused as the acidic aqueous solution.

(Evaluation)

0.2-g samples of the respective activated carbon products obtained inExamples C1 and C2 and Comparative Example C1 were subjected to wetdecomposition using 240 mL of nitric acid, and then 20 mL of perchioricacid. Afterward, the residual contents of heavy metals including nickel,iron and zinc were measured using an inductively coupled plasma emissionspectrometry analysis device (ICP analysis device, IRIS AP manufacturedby Thermo Electron Corporation). The results obtained are shown in Table3.

Furthermore, mixtures consisting of 81 wt % of activated carbonrespectively obtained in Examples C1 and C2 and Comparative Example C1,9 wt % of conductive carbon black (Denka Black, manufactured by DenkiKagaku Kogyo Co.) and 10 wt % of polytetrafluoroethylene (Teflon(registered trademark) 6J, manufactured by Mitsui-du Pont Chemical Co.)were kneaded. The kneaded mixtures thus obtained were molded into sheetswith a thickness of 200 μm by rolling, and were punched into circularshapes with a diameter of 11 mm using a punching device. Afterward,sheet-form polarizing electrodes were manufactured by drying thesepunched sheets at 200° C. under reduced pressure.

Using the polarizing electrodes thus obtained, a current collectingmember, polarizing electrode sheet, polypropylene non-woven fabric,polarizing electrode sheet and current collecting member were laminatedin this order in a stainless steel case (as shown in FIG. 1) inside aglove box with a dew point of −80° C. or less. Afterward, the polarizingelectrodes were impregnated with a propylene carbonate solutioncontaining 1 mole of tetraethylammonium tetrafluoroborate, and weresealed by crimping to the upper lid of the stainless steel case using aninsulating gasket made of polypropylene, thus producing electricaldouble layer capacitors.

The electrical double layer capacitors thus obtained were charged with aconstant current at 4 mA/cm² relative to the electrode surface area upto an attained voltage of 2.7 V at room temperature using an electricaldouble layer capacitor evaluation device manufactured by HIOKI E.E.CORPORATION, and were then subjected to supplementary charging atconstant voltage of 2.7 V until the charging current reached 1 mA/cm².Following the completion of this supplementary charging,constant-current charging was performed at 2 mA/cm², and the initialelectrostatic capacitance was determined. After this charge-dischargecycle was repeated 50 times, constant-current charging at 4 mA/cm²relative to the electrode surface area was again performed up to anattained voltage of 2.7 V, and supplementary charging was performed at aconstant voltage of 2.7 V until the charging current reached 1 mA/cm².Following the completion of this supplementary charging, the capacitorswere allowed to stand for 50 hours; then, a constant-current dischargewas performed at 2 m/cm², and the electrostatic capacitance afterstanding was determined. Then, the rate of self-discharge retention (%)was determined by dividing this electrostatic capacitance after standingby the previously determined initial electrostatic capacitance. Theresults obtained are shown in Table 3. TABLE 3 Comparative ExampleExample C1 C2 C1 Ni content (ppm) 8 6 22 Fe content (ppm) 0.3 0.2 0.6 Zncontent (ppm) 0.1 0.3 0.8 Overall heavy metal content 10.3 8.6 23.6(ppm) Initial electrostatic 33.2 33.7 32.6 capacitance (F/cc)Electrostatic capacitance 32.3 32.4 22.3 after standing (F/cc) Rate ofSelf-discharge 97.2 96.1 68.4 retention (%)

As is seen from Table 3, the electrical double layer capacitorsmanufactured from the activated carbon of Example C1 and Example C2, inwhich the overall heavy metal content was 20 ppm or less, showed therates of self-discharge retention of 90% or greater, and thus showedsuperior performance as electrical double-layer capacitors.

On the other hand, the electrical double layer capacitor manufacturedfrom the activated carbon of Comparative Example C1, in which theoverall heavy metal content exceeded 20 ppm, showed the rate ofself-discharge retention of less than 90%, and thus showed inadequateperformance as an electrical double layer capacitor.

Example D1

An optically anisotropic pitch with a Mettler softening point of 285° C.obtained by heat-treating a petroleum decomposition residue was spun bymelt blow-spinning using a nozzle which had 1000 spinning holes with adiameter of 0.2 mm in a slit having a width of 2 mm, and these spunfibers were subjected to an infusibilizing treatment and a carbonizationtreatment, thus producing mesophase pitch carbon fibers (manufactured byPetoka Materials).

The mesophase pitch carbon fibers thus obtained were pulverized to 0.02mm, and 200 g of 95% potassium hydroxide was added and mixed with 100 gof the pulverized fibers thus obtained. This mixture was placed in anickel reaction tube with a diameter of 4 inches in which a nitrogenintroduction tube and a off-gassing line were mounted. This reactiontube was set in a cylindrical furnace, and the temperature was elevatedto 700° C. at a temperature elevation rate of 3.3° C./minute whilenitrogen was caused to flow through at a rate of 100 milliliters(mL)/minute. The system was maintained at this temperature for 1 hour.Afterward, the system was cooled to room temperature at a rate of 5°C./minute, thus producing an activation treatment product.

The activation treatment product thus obtained was placed in apressurized filter with a diameter of 3 cm, and this product was washedat a pressure of 0.2 MPa using 2 liters (L) of ion exchange water at 80°C. Next, the product was washed at a pressure of 0.2 MPa using 2 L of 1N hydrochloric acid at 80° C. Furthermore, the product was then washedat a temperature of 60° C. and a pressure of 0.2 MPa using 4 L of ionexchange water, thus producing activated carbon. The activated carbonthus obtained was heated for 3 hours at 100° C., and was then heated for8 hours at a pressure of 0.1 MPa and dried under reduced pressure, thusproducing activated carbon in a dry state.

Example D2

Activated carbon was manufactured by the same operation as in ExampleD1, except that 2 N hydrochloric acid was used instead of the 1 Nhydrochloric acid used in Example D1.

Example D3

Activated carbon was manufactured by the same operation as in ExampleD1, except that the amount of ion exchange water at 80° C. that was usedwas changed from 2 L (in Example D1) to 1 L.

Comparative Example D1

Activated carbon was manufactured by the same operation as in ExampleD1, except that the washing with 1 N hydrochloric acid at 80° C. thatwas performed in Example D1 was performed at 20° C.

Comparative Example D2

Activated carbon was manufactured by the same operation as in ExampleD1, except that 4 N hydrochloric acid was used instead of the 1 Nhydrochloric acid used in Example D1.

Comparative Example D3

Activated carbon was manufactured by the same operation as in ExampleD1, except that the washing with ion exchange water at 80° C. that wasperformed in Example D1 was performed at 20° C.

(Evaluation)

0.2-g samples of the respective activated carbon products obtained inExamples D1 through D3 and Comparative Examples D1 through D3 weresubjected to wet decomposition using 240 mL of nitric acid, and then 20mL of perchloric acid. Afterward, the residual contents of heavy metalsincluding nickel were measured using an inductively coupled plasmaemission spectrometry analysis device (ICP analysis device, IRIS APmanufactured by Thermo Electron Corporation). Furthermore, the residualcontents of alkali metals including potassium metal were measured usingan atomic absorption analysis device (Deflecting Zeeman atomicabsorption photometer Z-5300, manufactured by Hitachi Seisakusho K.K.).The results obtained are shown in Table 4.

Furthermore, mixtures consisting of 81 wt % activated carbonrespectively obtained in Examples D1 through D3 and Comparative ExamplesD1 through D3, 9 wt % conductive carbon black (Denka Black, manufacturedby Denki Kagaku Kogyo Co.) and 10 wt % polytetrafluoroethylene (Teflon(registered trademark) 6J, manufactured by Mitsui-du Pont Chemical Co.)were kneaded. The kneaded mixtures thus obtained were molded into sheetswith a thickness of 200 μm by rolling, and were punched into circularshapes with a diameter of 11 mm using a punching device. Afterward,sheet-form polarizing electrodes were manufactured by drying thesepunched sheets at 200° C. under reduced pressure.

Using the polarizing electrodes thus obtained, a current collectingmember, polarizing electrode sheet, polypropylene non-woven fabric,polarizing electrode sheet and current collecting member were laminatedin this order in a stainless steel case (as shown in FIG. 1) inside aglove box with a dew point of −80° C. or less. Afterward, the polarizingelectrodes were impregnated with a propylene carbonate solutioncontaining 1 mole of tetraethylammonium tetrafluoroborate, and weresealed by crimping to the upper lid of the stainless steel case using aninsulating gasket made of polypropylene, thus producing electricaldouble layer capacitors.

The electrical double layer capacitors thus obtained were charged with aconstant current at 4 mA/cm² relative to the electrode surface area upto an attained voltage of 2.7 V at room temperature using an electricaldouble layer capacitor evaluation device manufactured by HIOKI E.E.CORPORATRION, and were then subjected to supplementary charging atconstant voltage of 2.7 V until the charging current reached 1 mA/cm².Following the completion of this supplementary charging,constant-current charging was performed at 2 mA/cm², and the initialelectrostatic capacitance was determined. After this charge-dischargecycle was repeated 50 times, constant-current charging at 4 mA/cm²relative to the electrode surface area was again performed up to anattained voltage of 2.7 V, and supplementary charging was performed at aconstant voltage of 2.7 V until the charging current reached 1 mA/cm².Following the completion of this supplementary charging, the capacitorswere allowed to stand for 50 hours; then, a constant-current dischargewas performed at 2 mA/cm², and the electrostatic capacitance afterstanding was determined. Then, the rate of self-discharge retention (%)was determined by dividing this electrostatic capacitance after standingby the previously determined initial electrostatic capacitance. Theresults obtained are shown in Table 4. TABLE 4 Example ComparativeExample D1 D2 D3 D1 D2 D3 Alkali metal content 83 92 182 183 124 273(ppm) K content (ppm) 82 91 181 183 123 272 Heavy metal content 14 13 1321 22 18 (ppm) Ni content (ppm) 7 4 11 18 21 12 Fe content (ppm) 6 7 1 10.5 4 Cu content (ppm) 0.1 0.1 0.1 0.2 0.1 0.2 Zn content (ppm) 0.1 0.10.1 0.15 0.08 0.25 Ag content (ppm) 0.05 0.02 0.06 0.09 0.02 0.07Initial 33.2 33.7 33.4 32.8 34.2 33.1 electrostatic capacitance (F/cc)Electrostatic 32.1 32.7 31.8 22.5 19.6 26.8 capacitance after standing(F/cc) Rate of Self- 96.6 97.0 95.3 68.5 57.3 81.2 discharge retention(%)

As is seen from Table 4, the electrical double layer capacitorsmanufactured from the activated carbon of Examples D1 through D3 inwhich the heavy metal content was 20 ppm or less and the alkali metalcontent was 200 ppm or less showed the rate of self-discharge retentionof 90% or greater, and thus showed superior performance as electricaldouble layer capacitors.

On the other hand, the electrical double layer capacitors manufacturedfrom the activated carbon of Comparative Examples D1 and D2, in whichthe alkali metal content did not exceed 200 ppm but the heavy metalcontent exceeded 20 ppm, and the electrical double layer capacitormanufactured from the activated carbon of Comparative Example D3, inwhich the heavy metal content did not exceed 20 ppm but the alkali metalcontent exceeded 200 ppm showed the rates of self-discharge retention ofless than 90%, and thus showed inadequate performance as electricaldouble layer capacitors.

Example D4

An activation treatment product obtained in the same manner as inExample D1 was placed in a pressurized filter with a diameter of 3 cm,and was washed at a pressure of 0.2 MPa with 2 liters (L) of ionexchange water at 80° C. Next, this product was washed at a pressure of0.2 MPa using 1 L of carbonated water with a carbonic acid concentrationof 1 wt % at 20° C. Next, the product was washed at a pressure of 0.2MPa with 2 L of a 1 N aqueous solution of hydrochloric acid at 80° C.Next, the product was washed at a pressure of 0.2 MPa with 0.5 L of 5%aqueous ammonia at 30° C. Finally, the product was washed with 3 L ofion exchange water at 60° C., and was dried to produce activated carbon.

Example D5

An activation treatment product obtained in the same manner as inExample D1 was placed in a pressurized filter with a diameter of 3 cm,and was washed at a pressure of 0.2 MPa with 2 liters (L) of ionexchange water at 80° C. Next, this product was washed at a pressure of0.2 MPa using 1 L of carbonated water with a carbonic acid concentrationof 1 wt % at 20° C. Next, the product was washed at a pressure of 0.2MPa with 2 L of a 1 N aqueous solution of hydrochloric acid at 80° C.Next, the product was washed at a pressure of 0.2 MPa with 0.5 L of 5%aqueous ammonia at 30° C., and was then further washed at a pressure of0.2 MPa with 0.5 L of a 1 N aqueous solution of hydrochloric acid at 80°C. Finally, the product was washed with 3 L of ion exchange water at 60°C., and was then dried to produce activated carbon.

(Evaluation)

0.2 g samples of the respective activated carbon products obtained inExamples D4 and D5 were subjected to wet decomposition using 240 mL ofnitric acid, and then 20 mL of perchloric acid. Afterward, the residualcontents of heavy metals including nickel were measured using aninductively coupled plasma emission spectrometry analysis device (ICPanalysis device, IRIS AP manufactured by Thermo Electron Corporation).Furthermore, the residual contents of alkali metals including potassiummetal were measured using an atomic absorption analysis device(Deflecting Zeeman atomic absorption photometer Z-5300, manufactured byHitachi Seisakusho K.K.). The results obtained are shown in Table 5.

Furthermore, electrical double layer capacitors were manufactured in thesame manner as in Example D1 using the activated carbon respectivelyobtained in Examples D4 and D5, and the initial electrostaticcapacitance (F/cc), electrostatic capacitance after standing (F/cc) andthe rate of self-discharge retention (%) were evaluated. The resultsobtained are shown in Table 5.

Furthermore, the carbon components extracted of the activated carbonobtained in Example D4 or D5 were measured as follows: specifically, 70g samples of the activated carbon obtained in Examples D4 and D5 wererespectively placed in a 1.5-L glass three-necked flask which wasequipped with an agitator, a cooling condenser and a thermometer, andwhich contained 700 g of toluene. The samples were agitated anddispersed, and were heated to reflux for 1 hour at 115° C., so that acarbon component extraction treatment was performed. After the liquidmixture was cooled to room temperature, the liquid mixture was subjectedto a filtration treatment, and the activated carbon that was filteredout was heated for 3 hours at 100° C., and was then dried under reducedpressure (heating for 8 hours at a pressure of 0.1 MPa), thus producingactivated carbon in a dry state. The extraction weight loss wasdetermined by comparing the weights of the activated carbon before andafter the extraction treatment. The results obtained are shown in Table5. TABLE 5 Example D4 D5 Alkali metal content 30 29 (ppm) K content(ppm) 26 28 Heavy metal content (ppm) 0.4 0.3 Ni content (ppm) 0.2 0.2Fe content (ppm) Below detection Below detection limit limit Cu content(ppm) Below detection Below detection limit limit Zn content (ppm) Belowdetection Below detection limit limit Ag content (ppm) Below detectionBelow detection limit limit Extracted carbon 0.09 0.08 component (wt %)Initial electrostatic 34.1 34.3 capacitance (F/cc) Electrostaticcapacitance 33.6 34.1 after standing (F/cc) Rate of Self-discharge 98.599.7 retention (%)

It was found that if washing is performed in the order of “hot water,carbonated water, aqueous ammonia, and hot water” (Example D4) or in theorder of “hot water, carbonated water, aqueous ammonia, hot hydrochloricacid and hot water” (Example D5) instead of in the washing order used inExamples D1 through D3 (hot water, hot hydrochloric acid and water),both the alkali metal content and heavy metal content show a great drop,and that (in particular) the rate of self-discharge retention can becaused to approach 100%.

INDUSTRIAL APPLICABILITY

In the activated carbon of the first invention of the presentapplication, the overall content of alkali metals is 100 ppm or less.Accordingly, in cases where this activated carbon is used as a rawmaterial in electronic devices, the formation of dendrites by thereductive deposition of alkali metals tends not to occur, so thatproblems such as short-circuiting, etc., tend not to occur. Furthermore,this activated carbon shows a good rate of self-discharge retention, andin cases where (for example) this activated carbon is used as anadsorbent material in the manufacture of clean water, the elution ofalkali metals into the clean water can be greatly suppressed, so thatthis activated carbon is suitable for use in applications relating tothe manufacture of food products or drugs, the manufacture of cleanwater, electronic devices or the like.

Furthermore, in the second and third inventions of the presentapplication, the overall content of heavy metals is 20 ppm or less.Accordingly, in cases where this activated carbon is used as a rawmaterial in electronic devices, the formation of dendrites by thereductive deposition of alkali metals tends not to occur, so thatproblems such as short-circuiting, etc., tend not to occur. Furthermore,this activated carbon shows a good rate of self-discharge retention.

In particular, in the activated carbon of the third invention of thepresent application, the alkali metal content is 200 ppm or less.Accordingly, in cases where this activated carbon is used as a rawmaterial in electronic devices, the formation of dendrites by thereductive deposition of alkali metals tends not to occur, so thatproblems such as short-circuiting, etc., tend not to occur. Furthermore,this activated carbon shows a good rate of self-discharge retention.Consequently, this activated carbon is suitable as a raw material foruse in electrical and electronic devices such as electrical double layercapacitors.

1. An activated carbon, obtained by subjecting a carbonaceous materialto an activation treatment, wherein the overall content of alkali metalsin said activated carbon is 100 ppm or less.
 2. The activated carbonaccording to claim 1, wherein said alkali metals are sodium and/orpotassium.
 3. An activated carbon manufacturing method, comprisingsubjecting a carbonaceous material to an activation treatment, and thenwashing the activation treatment product, thus obtained, with a liquidthat contains carbonic acid to give the activated carbon.
 4. Anactivated carbons, obtained by subjecting a carbonaceous material to anactivation treatment, wherein the overall content of heavy metals insaid activated carbon is 20 ppm or less.
 5. The activated carbonaccording to claim 4, wherein said heavy metals comprise at least onemetal selected from the group consisting of nickel, copper, zinc andiron.
 6. The activated carbon according to claim 5, wherein said heavymetals comprise at least nickel, and wherein the nickel content is 8 ppmor less.
 7. The activated carbon according to claim 5, wherein saidheavy metals comprise at least zinc, and wherein the zinc content is 1ppm or less.
 8. The activated carbon according to claim 5, wherein saidheavy metals comprise at least copper, and wherein the copper content is1 ppm or less.
 9. The activated carbon according to claim 5, whereinsaid heavy metals comprise at least iron, and wherein the iron contentis 0.3 ppm or less.
 10. An activated carbon manufacturing method,comprising subjecting a carbonaceous material to an activationtreatment, and then washing the activation treatment product, thusobtained, with a liquid containing a basic substance to give theactivated carbon.
 11. An activated carbon, obtained by subjecting aneasily graphitizable carbonaceous material to an alkali activationtreatment, wherein, in said activated carbon, the overall content ofheavy metals is 20 ppm or less, and the content of alkali metals is 200ppm or less.
 12. The activated carbon according to claim 11, whereinsaid heavy metals comprise at least one metal selected from the groupconsisting of nickel, copper, zinc and iron.
 13. The activated carbonaccording claim 11, wherein said heavy metals comprise at least nickel,and wherein the nickel content is 8 ppm or less.
 14. The activatedcarbon according to claim 11, wherein said heavy metals comprise atleast iron, and wherein the iron content is 0.3 ppm or less.
 15. Theactivated carbon according to claim 11, wherein said heavy metalscomprise at least zinc, and wherein the zinc content is 0.3 ppm or less.16. The activated carbon according to claim 11, wherein said heavymetals comprise at least copper, and wherein the copper content is 1 ppmor less.
 17. The activated carbon according to claim 11, wherein saidalkali metals are sodium and/or potassium.
 18. The activated carbonaccording to claim 11, comprising a silver content of 0.1 ppm or less.19. The activated carbon according to claim 11, wherein the carboncontents extracted by an extraction treatment using a hydrocarbonsolvent, is 0.2 wt % or less.
 20. An activated carbon manufacturingmethod, comprising subjecting an easily graphitizable carbonaceousmaterial to an alkali activation treatment, and then washing theactivation treatment product, thus obtained, with an acidic aqueoussolution containing an oxidizing agent to give the activated carbon. 21.The activated carbon manufacturing method according to claim 20, whereinan alkali metal hydroxide, used as an activation assistant in the alkaliactivation treatment, is sodium hydroxide and/or potassium hydroxide.22. The activated carbon manufacturing method according to claim 20,wherein said acidic aqueous solution is hydrochloric acid.
 23. Theactivated carbon manufacturing method according to claim 20, whereinsaid oxidizing agent is hydrogen peroxide.
 24. An activated carbonmanufacturing method, comprising subjecting an easily graphitizablecarbonaceous material to an alkali activation treatment, and thenwashing the activation treatment products thus obtained, with hot water,hot hydrochloric acid and water, in that order, to give the activatedcarbon.
 25. An activated carbon manufacturing method, comprisingsubjecting an easily graphitizable carbonaceous material to an alkaliactivation treatment, and then washing the activation treatment product,thus obtained, with hot water, carbonate water, hot hydrochloric acid,aqueous ammonia and hot waters in that order, to give the activatedcarbon.
 26. An activated carbon manufacturing method, comprisingsubjecting an easily graphitizable carbonaceous material to an alkaliactivation treatment, and then washing the activation treatment product,thus obtained, with hot water, carbonated water, hot hydrochloric acid,aqueous ammonia, hot hydrochloric acid and hot water, in that order, togive the activated carbon.
 27. The activated carbon manufacturing methodaccording to claim 24, wherein the alkali metal hydroxides that is usedas an activation assistant in the alkali activation treatments is sodiumhydroxide and/or potassium hydroxide.
 28. The activated carbonmanufacturing method according to claim 24, wherein the temperature ofsaid hot water is 30 to 95° C.
 29. The activated carbon manufacturingmethod according to claim 24, wherein the temperature of said hothydrochloric acid is 60 to 90° C.
 30. The activated carbon manufacturingmethod according to claim 24, wherein the concentration of said hothydrochloric acid is 0.5 to 3 N.
 31. A polarizing electrodes which isformed by mixing the activated carbon according to claim 1, with atleast a binder and a conductive material.
 32. An electrical double layercapacitor, comprising the polarizing electrode according to claim 31.